Does Vitamin D Make You A Better Athlete?

Most of us don't get enough, but it's not clear that holds back our athletic performance.

If, like many people, you don't get enough vitamin D from sun and diet, then you'll likely benefit from taking vitamin D supplements in a number of ways. But will it make you a better athlete? That's a claim that has been made frequently over the past few years, generally with flimsy-to-nonexistent evidence. So I was happy to see a recent paper that takes a more rigorous approach to testing the idea that "optimal" levels of vitamin could enhance athletic performance in typical Western athletes (as opposed to, say, kids with rickets). The study is in the current issue of Journal of Sports Sciences, from a group at Liverpool John Moores University in England.

The first part of the study has a simple aim: find out if athletes in the U.K., tested during the winter months, have lower vitamin D levels than non-athlete controls. They tested rugby players, soccer players, and jockeys (from both both flat horse racing and races involving jumps -- seems a little odd, but apparently the point was that jockeys are an example of athletes who often eat restricted diets in order to make weight, and thus are more likely to be deficient), along with a group of healthy non-athlete controls. The group sizes ranged from 11 soccer players to 18 jumping jockeys, and 30 controls. Here's the basic data:

Okay, so nothing particularly remarkable here. All the groups are pretty similar, though you could say the soccer players seem a little higher than the controls and the jockeys a little lower. The group sizes are too small to draw any serious group-vs-group conclusions. The question is: are these vitamin D levels adequate? That, as it turns out, is a very involved question. The US Institude of Medicine classifies people by 25(OH)D concentration as follows:

<12 = severely deficient

12-30 = deficient

30-50 = inadequate

>50 = adequate

But these categories have come under a lot of discussion. In the UK, anything above 25 is officially okay; on the other hand, others have argued that 100-250 should be considered "optimal." On the other other (third?) hand, a study last summer suggested that levels above 140 were associated with higher mortality risk. In other words, no one really knows at this point, but 50 seems like a reasonable minimal target -- and more than half of athletes and the control group in this study fell below that mark.

So the next question is, does boosting your levels above 50 make you a better athlete? To test that, the researchers asked the soccer players from the original study to undertake a double-blinded, placebo-controlled vitamin D supplementation study. Ten athletes took either a 5000 IU pill of vitamin D each day, or a placebo, for eight weeks (for comparison, the capsules that I currently have contain 1000 IU each). This significantly boosted 25(OH)D levels, to the point that three of the five athletes in the supplement group had levels over 100 by the end. The result: "There was a significant improvement in 10 m sprint times and vertical jump in the vitamin D group whereas the placebo showed no change."

My first reaction: this sounds really encouraging. When I looked more closely, though, the situation was a little less clear-cut. The study actually measured six different performance measures -- 10 m sprint, 30 m sprint, vertical jump, bench press, back squat, and something called the Illinois Agility Test -- but only saw significant improvements in two of them. This is always grounds for caution, because if you look at enough variables, you'll always find some that improve. And why, for example, did 10 m sprint improve whereas 30 m sprint didn't? Overall, the results are still intriguing, but (with just five subjects in each group) very preliminary.

And that's where things would have been left -- except that, as I was writing this up, I noticed that the same group (pretty much) had just published another study online-ahead-of-print in the British Journal of Sports Medicine, with a slightly different design. This time the subjects were 30 club-level university athletes (rugby, soccer, and others), and the supplementing protocol was either nothing, 20,000 IU per week, or 40,000 IU per week (compared to 7 x 5000 = 35,000 per week for the previous study). The duration of the study was 12 weeks, and the performance tests were similar: bench press, leg press, vertical jump, and 20 m sprint.

Once again, most of the subjects -- 57% of them -- started the study below 50 nmol/L, so nominally "inadequate." By halfway through the supplementation period, every subject had boosted their levels above 50. In fact, levels mostly stabilized after that (and even decreased between 6 and 12 weeks in the higher supplementation group). By the end, there was no difference between the 20,000 and 40,000 IU groups.

And the performance tests? No difference in any of the groups -- not even a hint of a "nonsignificant trend" for improvement, despite the dramatic increases in vitamin D levels. So what does this mean? Well, my first reaction is that it makes me less confident in the results of the much smaller previous study. If you're a big vitamin D booster, you might suggest (as the authors do) that they needed an even higher dose to get their levels above 100 (the average of the two supplement groups was ~85-90 at the end of the study). Or maybe there was something in the characteristics of the subjects that made them less likely to improve. Or perhaps vitamin D, even though it appears to have important long-term health effects, and even though there's biological evidence that vitamin plays a role in muscle function, simply isn't a limiting factor in real-world athletic performance at the levels typically found in northern hemisphere athletes. At this point, that's my assumption -- although personally, I'll continue taking vitamin D on a casual basis, especially during the winter, for general health.